In FW, fish usually maintain plasma osmolality at ~ 281–310 mOsm kg− 1 (Nordlie, 2009), significantly higher than that of their surroundings (< 50 mOsm kg− 1).
In SW, plasma osmolalities of ~ 400 mOsm kg− 1 are observed (Nordlie, 2009), significantly lower than the osmolality of the environment (~ 1050 mOsm kg− 1).
Therefore life in most waters requires fish to maintain an osmotic balance, pumping ions against the concentration gradient, either into the body in FW, or out of the body in SW.isosmotic point.